1. Field of the Invention
The present invention relates to an image formation apparatus and an adjustment method thereof. More particularly, the present invention relates to an image formation apparatus such as a copying machine and a printer that allows correction of .gamma. correction data.
2. Description of the Related Art
A digital image formation apparatus such as a laser printer, a digital copying machine, and the like that drives a light source according to image data converted into a digital value for forming an image is widely used. Various digital image formation apparatuses for reproducing the so-called half tone image such as a photograph in fidelity are proposed.
Such digital image formation apparatuses employ the method of area gradation using a dither matrix, or the multi-level laser exposure method for representing the gradation tone of one printed dot by varying the pulse width or the light emission intensity of the laser to alter the laser light. A digital image formation apparatus employing a multi-level dither method which is a combination of a dither and pulse width modulation system or an intensity modulation system is also known.
According to these types of gradation methods, the image density having a gradation tone proportional to the gradient of the image data to be reproduced should be obtained in principle. However, in practice, the original density to be reproduced and the actually reproduced image density do not correspond accurately with each other due to the complicated relationship of various factors such as the photosensitive characteristics of the photoconductor, the characteristics of the toner, the used environment, and the like. These characteristics are generally referred to as .gamma. characteristics. The .gamma. characteristics become a critical factor in reducing the fidelity of the reproduced image particularly for half tone originals.
Therefore, .gamma. correction is carried out to improve the fidelity of the reproduced image. The read out original density is converted using a conversion table in which predetermined data for .gamma. correction is stored. A digital image is formed according to the converted original density wherein the original density and the image density meet a linear relationship. By generally applying .gamma. correction, an image corresponding to the original density can be reduced in fidelity.
A conventional digital image formation apparatus that carries out the above-described .gamma. correction provides density control by altering at least the photoconductor surface potential prior to an exposure operation or the development bias potential applied in advance to a developer unit. Also, gradation variation is compensated for by selecting a predetermined one of a plurality of .gamma. correction conversion tables prestored with .gamma. correction data in response to change in the gradation tone corresponding to variation in the photoconductor surface potential and development bias potential to control the amount of light of the exposure device in a non-linear manner.
In the above-described conventional digital image formation apparatus, .gamma. correction is carried out using a predetermined number of .gamma. correction conversion tables prepared in advance. Therefore, fine correction corresponding to change in the environment and the like cannot be applied. There was a problem that the optimum .gamma. correction is not always applied.
The .gamma. correction conversion table to be used must have a great amount of data corresponding to an enormous amount of combination patterns stored therein when each of the plurality of factors varies which causes change in the .gamma. curve. A memory of a great capacity for storing the .gamma. correction conversion table is required. In a fixed type full color copying machine, the aforementioned combination pattern becomes more versatile due to a different developing position for each color than in the case of a monochrome copying machine or an elevator type full color copying machine. An elevator type full color copying machine includes development units of appropriate colors arranged vertically to carry out development by moving the development units vertically by selectively setting one of the development units opposite the photoconductor. The development position is always the same irrespective of which color is to be developed. A fixed type full color copying machine has the development units of appropriate colors sequentially arranged around the circumference of the photoconductor. Development of each color is carried out by selectively actuating a development unit. Therefore, the development position differs from color to color.
Usage of a memory of a great capacity for storing a .gamma. correction conversion table corresponding to the huge amount of combination patterns will increase the cost of the apparatus. Also, each combination pattern must be subjected to experiments to produce a .gamma. correction conversion table corresponding to each combination pattern. The number of steps for the experiments carried out is significant. There was a problem that development cannot be carried out efficiently.